New Accounting Pronouncements (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended									12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncement, Early Adoption [Line Items]
Paid-in capital		$ 6,688.5				$ 6,635.3					$ 6,688.5	$ 6,635.3
Income tax benefit (expense)											53.2	(1,055.2)	$ (61.5)
Net cash provided by (used in) operating activities											1,866.3	1,126.9	715.9
Other current assets, net		277.6				210.7					277.6	210.7
Other assets		681.5				382.5					681.5	382.5
Net Cash Provided by (Used in) Financing Activities											(1,496.4)	11,317.9	(531.5)
Net income (loss) attributable to Molson Coors Brewing Company		$ 588.8	$ 287.0	$ 329.9	$ 208.5	$ 1,439.3	$ 207.7	$ 177.9	$ 168.1		$ 1,414.2	$ 1,993.0	$ 395.2
Basic net income (loss) attributable to Molson Coors Brewing Company per share		$ 2.73	$ 1.33	$ 1.53	$ 0.97	$ 6.70	$ 0.97	$ 0.83	$ 0.83		$ 6.57	$ 9.40	$ 2.13
Long-term debt		$ 10,598.7				$ 11,387.7					$ 10,598.7	$ 11,387.7
Earnings Per Share, Diluted		$ 2.72	$ 1.33	$ 1.52	$ 0.96	$ 6.65	$ 0.96	$ 0.82	$ 0.82		$ 6.53	$ 9.34	$ 2.12
Weighted-average shares for diluted EPS											216.5	213.4	186.4
Retained earnings		$ 7,206.1				$ 6,145.3					$ 7,206.1	$ 6,145.3
Current portion of long-term debt and short-term borrowings		714.8				684.8					714.8	684.8
As Reported
New Accounting Pronouncement, Early Adoption [Line Items]
Income tax benefit (expense)		392.4	$ (145.3)	$ (123.0)	$ (64.6)	(993.2)	$ (19.6)	$ (21.2)	$ (16.7)		(59.5)	1,050.7	$ 51.8
Other assets		681.5	508.4	453.6	426.1	382.5	272.8	282.8	264.1		681.5	382.5
Net income (loss) attributable to Molson Coors Brewing Company		$ 580.4	$ 280.0	$ 323.3	$ 201.3	$ 1,438.4	$ 202.5	$ 172.3	$ 162.7		$ 1,385.0	$ 1,975.9	$ 359.5
Basic net income (loss) attributable to Molson Coors Brewing Company per share		$ 2.69	$ 1.30	$ 1.50	$ 0.94	$ 6.70	$ 0.94	$ 0.80	$ 0.80		$ 6.43	$ 9.32	$ 1.94
Earnings Per Share, Diluted		$ 2.68	$ 1.29	$ 1.49	$ 0.93	$ 6.65	$ 0.94	$ 0.80	$ 0.80		$ 6.40	$ 9.26	$ 1.93
Retained earnings		$ 7,150.6	$ 6,658.7	$ 6,467.0	$ 6,232.0	$ 6,119.0	$ 4,768.9	$ 4,654.5	$ 4,570.4		$ 7,150.6	$ 6,119.0
As Adjusted
New Accounting Pronouncement, Early Adoption [Line Items]
Income tax benefit (expense)		391.7	(147.4)	(125.2)	(65.9)	(996.3)	(20.1)	(21.7)	(17.1)		(53.2)	1,055.2	$ 61.5
Other assets		681.5	522.3	462.6	430.4	382.5	278.7	286.8	266.2		681.5	382.5
Net income (loss) attributable to Molson Coors Brewing Company		$ 588.8	$ 287.0	$ 329.9	$ 208.5	$ 1,439.3	$ 207.7	$ 177.9	$ 168.1		$ 1,414.2	$ 1,993.0	$ 395.2
Basic net income (loss) attributable to Molson Coors Brewing Company per share		$ 2.73	$ 1.33	$ 1.53	$ 0.97	$ 6.70	$ 0.97	$ 0.83	$ 0.83		$ 6.57	$ 9.40	$ 2.13
Earnings Per Share, Diluted		$ 2.72	$ 1.33	$ 1.52	$ 0.96	$ 6.65	$ 0.96	$ 0.82	$ 0.82		$ 6.53	$ 9.34	$ 2.12
Retained earnings		$ 7,206.1	$ 6,705.8	$ 6,507.1	$ 6,265.5	$ 6,145.3	$ 4,794.3	$ 4,674.7	$ 4,585.0		$ 7,206.1	$ 6,145.3
Cost of Goods Sold [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											19.0
Selling, General and Administrative Expenses [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											20.0
Special Items, Net [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											8.0
Other income (expense), net
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											47.0
Minimum
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification										$ 70.0
Cumulative Effect on Retained Earnings, Net of Tax	$ 26.0
Maximum
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification										$ 90.0
Cumulative Effect on Retained Earnings, Net of Tax	$ 30.0
Europe Segment [Member] | Cost of Goods Sold [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											27.0
Europe Segment [Member] | Selling, General and Administrative Expenses [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											19.0
U.S. Segment & Canada Segment [Member] | Selling, General and Administrative Expenses [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											0.5
Canada | Cost of Goods Sold [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											1.0
Canada | Special Items, Net [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											3.0
United States Segment [Member] | Cost of Goods Sold [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											7.0
United States Segment [Member] | Special Items, Net [Member]
New Accounting Pronouncement, Early Adoption [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification											$ 5.0